| Mass unit kg quantum metrology redefinition based on the Planck constant h,has been recognized as a challenging research in the field of international metrology.In 2012,it was listed as one of the world's six tough scientific problems by Nature magazine.Joule balance is the“Chinese solution”of the mass quantum metrology independently proposed by the National institution of Metrology,China.The principle of the joule balance is to establish the relationship between the Planck constant h,the electrical quantum benchmarks and the mass by a balance of the mechanical energy and the electromagnetic energy,and realize the mass quantum metrology indirectly.The standard mass is derived from the balance relationship between its gravity and an electromagnetic force,which requires the abovementioned two force vectors to be aligned along the direction of gravity.Otherwise,the force alignment error will be introduced to the joule balance experiment,which will affect the accuracy of the mass quantum metrology.The latest results show that,the relative uncertainty of force alignment is 9×10-8,which is one of the biggest components to total relative uncertainty of joule balance experiment.Therefore,it is necessary to study the force vector alignment of joule balance to improve its accuracy.These studies will provide strong technical supports for realizing the mass quantum metrology benchmark by joule balance.By researching the coupling relationship between the electromagnetic force vector and the standard gravity vector,this paper proposes a decoupling method based on an overlapping layout of force acting points.According to the analysis of the force vector alignment errors of joule balance,an electromagnetic force vector alignment adjustment method based on an absolute deviation solution model and a standard gravity vector self-alignment method based on an overlapped-flexure hinge are proposed,respectively.In order to verify theory and method proposed in this paper,force alignment experiments are carried out.The main research work is as follows:As the coupling between the electromagnetic force vector and the standard gravity vector due to misaligned parasitic torque,a decoupling method based on an overlapping layout of force acting points is proposed.On the one hand,the overlapping layout of acting points of the electromagnetic force vector and standard gravity vector to avoid the misaligned parasitic torque generation.On the other hand,based on the principle of systematic force vector balance mechanics,a force decoupling model is established,then the stiffness of the flexure hinge is obtained when force alignment,which provides theoretical support for flexure hinge design.The decoupling method not only meets the decoupling requirements of force alignment,but also reduces the influence between the electromagnetic force and the standard gravity.Verification experiments are carried out using the new designed suspension system.The results show that,the coupling effect of the standard gravity vector to the electromagnetic force vector is reduced by 71.2%in mean.As the electromagnetic force vector is difficult to be aligned due to the strong coupling between the electromagnetic parasitic forces and parasitic torques of joule balance,an adjusting method based on the absolute deviation solution model is proposed.Firstly,according to Ampere's law and the mechanics vector balance principle,the description models of the electromagnetic force vector alignment errors are established separately,then the relationship between the relative changes of the suspension coil and the absolute deviation of the electromagnetic force in the misaligned state is established.The absolute deviation of the coil can be obtained by the proposed model.Secondly,a suspension system with measuring and adjusting functions is designed.Specifically,the posture of suspension coil is determined by 6-axis laser interferometer and 2-axis position sensitive detectors(PSDs).The horizontal deviations is eliminated by adjusting the precision translation stage and the tilt deviations is eliminated by changing the length between the three hang rods using piezoelectric ceramics.The results of verification experiments show that,the alignment relative uncertainty of electromagnetic force vector is reduced from2.6×10-8 to 4.2×10-9.As the standard gravity vector is difficult to be aligned due to the combined effect of deviation torque and eccentric load torque,a self-alignment adjustment method based on an overlapped-flexure hinge is proposed.Firstly,the standard gravity vector deviation torque is eliminated by the overlapped-flexure structure.Secondly,the stiffness model of the overlapped-flexure hinge is established based on Castigliano's second theorem,and the key sizes of the notch are designed according to the actual loads on the suspension system.The stiffness of the hinge can meet the requirements of standard gravity vector self-alignment.When a standard mass(1kg)is loaded in the decoupling range of the flexure hinge,the eccentric load torque can be eliminated.The results of verification experiments show that,the relative change of horizontal displacement of the suspension coil is less than 1.8?m,and the relative change of tilt angle of the suspension coil is less than 0.8?rad,thus the gravity of the standard mass has been achieved alignment.At last,according to the adjusting methods proposed in this paper,the electromagnetic force and gravity vector are loaded on the new suspension system.The experimental results show that,the relative change of the horizontal displacement of the suspension coil is less than 3.6?m,and the relative change of the tilt angle of the suspension coil is less than 6.4?rad.The relative uncertainty of the force alignment is reduced from 9×10-8 to 1.3×10-8,which has met the force alignment requirement of the joule balance;meanwhile,the measurement capability of NIM-2 joule balance have been increased from 500 g to 1kg. |
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